A RF electrode for generating, plasma in a plasma chamber comprising an optical feedthrough. A plasma chamber comprising an RF electrode and a counter-electrode with a substrate support for holding a substrate, wherein a high-frequency alternating field for generating the plasma can be formed between the RF electrode and the counter-electrode. The chamber comprising an RF electrode with an optical feedthrough. A method, for in situ analysis or in situ processing of a layer or plasma in a plasma chamber, wherein the layer is disposed on counter-electrode and an RF electrode is disposed on the side lacing the layer. Selection of an RF electrode having an optical feedthrough, and at least one step in which electromagnetic radiation is supplied through the optical feedthrough for purposes of analysis or processing of the layer or the plasma, and by at least one other step in which the scattered or emitted or reflected radiation is supplied to an analysis unit.